Muridae (order Rodentia, suborder Myomorpha) A family of Old World rats and mice that are perhaps the most successful of all mammalian families. They are small, terrestrial, arboreal, burrowing, or semi-aquatic animals. The tail is long and scaly, the limbs pentadactyl and the first digit of the fore limb rudimentary. In some saltatorial forms the hind limbs are modified. Apodemus sylvaticus is the European field mouse. Mus musculus is the house mouse, which originated in the drier areas of Eurasia but has spread throughout the world. The Muridae appeared first during the Oligocene and radiated explosively during the Pliocene. They reached Australasia (including New Guinea and the Solomon Islands) before the first humans, and radiated there into some 28 genera, including Notomys (jerboa-rat), which resembles the true jerboas (family Dipodidae). There are at least 220 genera, with more than 1000 species.

Cricetidae (order Rodentia, suborder Myomorpha) A large and very successful family that comprises the hamsters, voles, lemmings, gerbils, and the New World rats and mice. The family is almost certainly paraphyletic. Most are terrestrial (and some of these are burrowing) but others are semi-aquatic. Their form varies according to their adaptation to a particular way of life. Peromyscus leucopus is the white-footed deer mouse of N. America, which bears a striking resemblance to the field mouse (Apodemus sylvaticus) of Europe, which belongs to the Muridae. The family also includes Mesocricetus auratus (golden hamster), and Ondatra zibethicus (musk-rat or musquash). Their distribution is world-wide except for Australasia and Malaysia. There are about 97 genera, with some 567 species.